Solution-based approach to make porous coatings for sinter-resistant catalysts

ABSTRACT

Catalyst systems that are resistant to high-temperature sintering and methods for preparing such catalyst systems that are resistant to sintering at high temperatures are provided. Methods of forming such catalyst systems include contacting a support having a surface including a catalyst particle with a solution comprising a metal salt and having an acidic pH. The metal salt is precipitated onto the surface of the support. Next, the metal salt is calcined to selectively generate a porous coating of metal oxide on the surface of the support distributed around the catalyst particle.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.15/399,179 filed on Jan. 5, 2017. The entire disclosure of the aboveapplication is incorporated herein by reference.

INTRODUCTION

This section provides background information related to the presentdisclosure which is not necessarily prior art.

The present disclosure relates to catalysts that are resistant tosintering at high temperatures and improved methods for preparingcatalysts that are resistant to sintering at high temperatures.

Metal nanoparticles can make up the active sites of catalysts used in avariety of applications, such as for the production of fuels, chemicalsand pharmaceuticals, and for emissions control from automobiles,factories, and power plants. Because metal nanoparticles tend toagglomerate, this decreases their surface area and active siteaccessibility, so they are often coupled to support materials. Thesupports physically separate the metal nanoparticles to preventagglomeration, and to increase their surface area and active siteaccessibility. Thus, catalyst systems typically include one or morecatalyst compounds; a porous catalyst support material; and one or moreoptional activators.

After continued use, especially at elevated temperatures, catalystsystems including supported metal particles lose catalytic activity dueto sintering, e.g., thermal deactivation that occurs at hightemperatures. Through various mechanisms, sintering results in changesin metal particle size distribution over a support and an increase inmean particle size; hence, a decrease in surface area for the activecatalyst compounds. For example, particle migration and coalescence is aform of sintering where particles of metal nanoparticles move or diffuseacross a support surface, or through a vapor phase, coalesce withanother nanoparticle, leading to nanoparticle growth. Ostwald ripeningis another form of sintering where migration of mobile species aredriven by differences in free energy and local atom concentrations on asupport surface. After sintering processes occur, catalyst activity candecrease. Therefore, catalyst systems are often loaded with a sufficientamount of supported catalyst metal particles to account for a loss ofcatalytic activity over time and to continue to have the ability tomeet, for example, emissions standards over a long period of operationat high temperatures.

Various techniques have been employed to decrease sintering of metalnanoparticle catalysts. For example, metals have been alloyed with othermetals, metal nanoparticles have been encapsulated with amorphouscoatings by, for example, atomic layer deposition, and strong metalnanoparticle anchoring on supports have been attempted. However, thesechemistry-based techniques have resulted in only limited success.Accordingly, there remains a need for improved catalysts that aresinter-resistant.

SUMMARY

This section provides a general summary of the disclosure, and is not acomprehensive disclosure of its full scope or all of its features.

In various aspects, the present disclosure provides a method ofpreparing a catalyst system that is resistant to sintering. The methodmay include contacting a support having a surface including a catalystparticle with a solution including a metal salt and having an acidic pH.The metal salt may be precipitated onto the surface of the support. Themetal salt may be calcined to selectively generate a porous coating ofmetal oxide on the surface of the support distributed around thecatalyst particle.

In one variation, the solution including the metal salt is aqueous andthe pH is less than or equal to about 6.

In one variation, the catalyst particle includes a metal selected fromthe group consisting of: platinum (Pt), ruthenium (Ru), rhodium (Rh),palladium (Pd), osmium (Os), iridium (Ir), gold (Au), iron (Fe), nickel(Ni), manganese (Mn), and combinations thereof.

In one variation, the catalyst particle includes an element selectedfrom the group consisting of: sodium (Na), potassium (K), magnesium(Mg), calcium (Ca), barium (Ba), cerium (Ce), lanthanum (La), phosphorus(P), and combinations thereof.

In one variation, the support includes a metal oxide selected from thegroup consisting of: cerium oxide (CeO₂), aluminum oxide (Al₂O₃),zirconium oxide (ZrO₂), titanium dioxide (TiO₂), silicon dioxide (SiO₂),magnesium oxide (MgO), zinc oxide (ZnO), barium oxide (BaO), potassiumoxide (K₂O), sodium oxide (Na₂O), calcium oxide (CaO), lanthanum oxide(La₂O₃), and combinations thereof.

In one variation, the metal salt includes an element selected from thegroup consisting of: aluminum (Al), cerium (Ce), zirconium (Zr),titanium (Ti), silicon (Si), magnesium (Mg), zinc (Zn), sodium (Na),potassium (K), barium (Ba), calcium (Ca), and combinations thereof.

In one variation, the metal salt is selected from the group consistingof: aluminum chloride (AlCl₃), aluminum nitrate (Al(NO₃)₃), aluminumhydroxide (Al(OH)₃), aluminum sulfate (Al₂(SO₄)₃), aluminum chlorate(Al(ClO₃)₃), aluminum phosphate (AlPO₄), aluminum metaphosphate(Al(PO₃)₃), and combinations thereof.

In one variation, prior to contacting, the method further includeswashing the surface of the support including the catalyst particle withan acidic solution.

In one variation, prior to the contacting, the method further includesdisposing the support having the surface including the catalyst particlein a reducing atmosphere to promote conversion of the catalyst particleto a metallic state.

In one variation, the disposing the support including the catalystparticle in the reducing atmosphere further includes disposing thesupport including the catalyst particle in a furnace. The furnace isthen purged with a gas mixture comprising an inert gas and hydrogen (H₂)at less than or equal to about 3% by volume for greater than or equal toabout 30 minutes. The support including the catalyst particle is heatedin the furnace having a temperature of greater than or equal to about200° C. to less than or equal to about 500° C. The method also includesmaintaining the support including the catalyst particle at thetemperature for greater than or equal to about 30 minutes and coolingthe support including the catalyst particle to ambient conditions.

In one variation, heating the support including a catalyst particleoccurs at a rate of less than or equal to about 20° C. per minute.

In one variation, the purging includes purging the furnace with the gasmixture having a flow rate of greater than or equal to about 1 standardcubic feet per hour (SCFH).

In one variation, the contacting of the surface of support including thecatalyst particle with the solution includes submerging the surface inthe solution. The method further includes applying ultrasound to thesolution to facilitate the precipitating.

In one variation, the contacting of the surface of support including thecatalyst particle with the solution includes submerging the surface inthe solution. The method further includes applying mixing the solutionwith a planetary centrifugal mixer to facilitate the precipitating.

In one variation, the precipitating the metal salt onto the surface ofthe support includes drying the solution including the metal salt tofacilitate the precipitation.

In one variation, the drying occurs at a temperature of greater than orequal to about 50° C. and at a pressure of less than or equal to about 1Torr. The drying process may be conducted for a time of greater than orequal to about 4 hours.

In one variation, the calcining the metal salt to selectively generate aporous coating of metal oxide on the support includes heating the metalsalt and the catalyst compound disposed on the support at greater thanor equal to about 400° C. to less than or equal to about 600° C. forgreater than or equal to about 2 hours.

In other aspects, the present disclosure provides a method of preparinga sinter-resistant catalyst system. The method optionally includescontacting a plurality of support particles each including a surfacebearing at least one catalyst particle with a liquid including a metalsalt and having a pH of less than or equal to about 6. Then, the metalsalt is precipitated onto the surface bearing the at least one catalystparticle. The method also includes calcining the metal salt toselectively generate a porous coating of metal oxide on the surfacebearing the at least one catalyst particle. The porous coating isdistributed around the at least one catalyst particle on the surface.

In one variation, prior to the contacting, the method further includesdisposing the support having the surface including the catalyst particlein a reducing atmosphere to promote conversion of the catalyst particleto a metallic state.

In one variation, the disposing the support including the catalystparticle in the reducing atmosphere further includes disposing thesupport including the catalyst particle in a furnace. The furnace isthen purged with a gas mixture comprising an inert gas and hydrogen (H₂)at less than or equal to about 3% by volume for greater than or equal toabout 30 minutes. The support including the catalyst particle is heatedin the furnace having a temperature of greater than or equal to about200° C. to less than or equal to about 500° C. The method also includesmaintaining the support including the catalyst particle at thetemperature for greater than or equal to about 30 minutes and coolingthe support including the catalyst particle to ambient conditions.

In yet other aspects, the present disclosure provides a catalyst systemincluding a platinum group metal catalyst bound to a metal oxidesupport. A crystalline coating of metal oxide nanoparticles isselectively disposed on the metal oxide support around the platinumgroup metal catalyst. The crystalline coating has a porosity of greaterthan about 20% to less than about 70% and covers greater than or equalto about 1.5% to less than or equal to about 80% of the surface area ofthe exposed surface, excluding the regions bound to the platinum groupmetal catalyst, of the metal oxide support.

The catalyst may include a metal selected from the group consisting of:platinum (Pt), ruthenium (Ru), rhodium (Rh), palladium (Pd), osmium(Os), iridium (Ir), gold (Au), iron (Fe), nickel (Ni), manganese (Mn),and combinations thereof. In variations where the catalyst is a platinumgroup metal catalyst, a metal may be selected from the group consistingof: platinum (Pt), ruthenium (Ru), rhodium (Rh), palladium (Pd), osmium(Os), iridium (Ir), gold (Au), and combinations thereof. The supportincludes a metal oxide selected from the group consisting of: ceriumoxide (CeO₂), aluminum oxide (Al₂O₃), zirconium oxide (ZrO₂), titaniumdioxide (TiO₂), silicon dioxide (SiO₂), magnesium oxide (MgO), zincoxide (ZnO), barium oxide (BaO), potassium oxide (K₂O), sodium oxide(Na₂O), calcium oxide (CaO), lanthanum oxide (La₂O₃), and combinationsthereof; and the metal oxide nanoparticles include aluminum oxide(Al₂O₃).

Further areas of applicability will become apparent from the descriptionprovided herein. The description and specific examples in this summaryare intended for purposes of illustration only and are not intended tolimit the scope of the present disclosure.

DRAWINGS

The drawings described herein are for illustrative purposes only ofselected embodiments and not all possible implementations, and are notintended to limit the scope of the present disclosure.

FIG. 1 is an illustration of a sinter-resistant monolithic catalystsystem prepared according to certain aspects of the present technology.

FIG. 2 is an illustration of a sinter-resistant particulate catalystsystem prepared according to certain aspects of the present technology.

FIG. 3 is a graph showing the lightoff temperatures for a coatedpowdered Pt—Pd catalyst as compared to a non-coated powdered Pt—Pdcatalyst.

FIG. 4 is a graph showing the lightoff temperature at 50% conversion fora coated monolith Pt—Pd catalyst as compared to a non-coated monolithPt—Pd catalyst.

FIG. 5 is a graph showing the lightoff temperature at 90% conversion fora coated monolith Pt—Pd catalyst as compared to a non-coated monolithPt—Pd catalyst.

Corresponding reference numerals indicate corresponding parts throughoutthe several views of the drawings.

DETAILED DESCRIPTION

Example embodiments are provided so that this disclosure will bethorough, and will fully convey the scope to those who are skilled inthe art. Numerous specific details are set forth such as examples ofspecific compositions, components, devices, and methods, to provide athorough understanding of embodiments of the present disclosure. It willbe apparent to those skilled in the art that specific details need notbe employed, that example embodiments may be embodied in many differentforms and that neither should be construed to limit the scope of thedisclosure. In some example embodiments, well-known processes,well-known device structures, and well-known technologies are notdescribed in detail.

The terminology used herein is for the purpose of describing particularexample embodiments only and is not intended to be limiting. As usedherein, the singular forms “a,” “an,” and “the” may be intended toinclude the plural forms as well, unless the context clearly indicatesotherwise. The terms “comprises,” “comprising,” “including,” and“having,” are inclusive and therefore specify the presence of statedfeatures, elements, compositions, steps, integers, operations, and/orcomponents, but do not preclude the presence or addition of one or moreother features, integers, steps, operations, elements, components,and/or groups thereof. Although the open-ended term “comprising,” is tobe understood as a non-restrictive term used to describe and claimvarious embodiments set forth herein, in certain aspects, the term mayalternatively be understood to instead be a more limiting andrestrictive term, such as “consisting of” or “consisting essentially of”Thus, for any given embodiment reciting compositions, materials,components, elements, features, integers, operations, and/or processsteps, the present disclosure also specifically includes embodimentsconsisting of, or consisting essentially of, such recited compositions,materials, components, elements, features, integers, operations, and/orprocess steps. In the case of “consisting of,” the alternativeembodiment excludes any additional compositions, materials, components,elements, features, integers, operations, and/or process steps, while inthe case of “consisting essentially of,” any additional compositions,materials, components, elements, features, integers, operations, and/orprocess steps that materially affect the basic and novel characteristicsare excluded from such an embodiment, but any compositions, materials,components, elements, features, integers, operations, and/or processsteps that do not materially affect the basic and novel characteristicscan be included in the embodiment.

Any method steps, processes, and operations described herein are not tobe construed as necessarily requiring their performance in theparticular order discussed or illustrated, unless specificallyidentified as an order of performance. It is also to be understood thatadditional or alternative steps may be employed, unless otherwiseindicated.

When a component, element, or layer is referred to as being “on,”“engaged to,” “connected to,” or “coupled to” another element or layer,it may be directly on, engaged, connected or coupled to the othercomponent, element, or layer, or intervening elements or layers may bepresent. In contrast, when an element is referred to as being “directlyon,” “directly engaged to,” “directly connected to,” or “directlycoupled to” another element or layer, there may be no interveningelements or layers present. Other words used to describe therelationship between elements should be interpreted in a like fashion(e.g., “between” versus “directly between,” “adjacent” versus “directlyadjacent,” etc.). As used herein, the term “and/or” includes any and allcombinations of one or more of the associated listed items.

Although the terms first, second, third, etc. may be used herein todescribe various steps, elements, components, regions, layers and/orsections, these steps, elements, components, regions, layers and/orsections should not be limited by these terms, unless otherwiseindicated. These terms may be only used to distinguish one step,element, component, region, layer or section from another step, element,component, region, layer or section. Terms such as “first,” “second,”and other numerical terms when used herein do not imply a sequence ororder unless clearly indicated by the context. Thus, a first step,element, component, region, layer or section discussed below could betermed a second step, element, component, region, layer or sectionwithout departing from the teachings of the example embodiments.

Spatially or temporally relative terms, such as “before,” “after,”“inner,” “outer,” “beneath,” “below,” “lower,” “above,” “upper,” and thelike, may be used herein for ease of description to describe one elementor feature's relationship to another element(s) or feature(s) asillustrated in the figures. Spatially or temporally relative terms maybe intended to encompass different orientations of the device or systemin use or operation in addition to the orientation depicted in thefigures.

Throughout this disclosure, the numerical values represent approximatemeasures or limits to ranges to encompass minor deviations from thegiven values and embodiments having about the value mentioned as well asthose having exactly the value mentioned. Other than in the workingexamples provided at the end of the detailed description, all numericalvalues of parameters (e.g., of quantities or conditions) in thisspecification, including the appended claims, are to be understood asbeing modified in all instances by the term “about” whether or not“about” actually appears before the numerical value. “About” indicatesthat the stated numerical value allows some slight imprecision (withsome approach to exactness in the value; approximately or reasonablyclose to the value; nearly). If the imprecision provided by “about” isnot otherwise understood in the art with this ordinary meaning, then“about” as used herein indicates at least variations that may arise fromordinary methods of measuring and using such parameters. For example,“about” may comprise a variation of less than or equal to 5%, optionallyless than or equal to 4%, optionally less than or equal to 3%,optionally less than or equal to 2%, optionally less than or equal to1%, optionally less than or equal to 0.5%, and in certain aspects,optionally less than or equal to 0.1%.

In addition, disclosure of ranges includes disclosure of all values andfurther divided ranges within the entire range, including endpoints andsub-ranges given for the ranges. As referred to herein, ranges are,unless specified otherwise, inclusive of endpoints and includedisclosure of all distinct values and further divided ranges within theentire range. Thus, for example, a range of “from A to B” or “from aboutA to about B” is inclusive of A and B.

Example embodiments will now be described more fully with reference tothe accompanying drawings.

Chemistry-based approaches for stabilizing metal nanoparticles have beenmet with limited success. Accordingly, the present technology provides asolution-based approach for minimizing or eliminating the sinteringprocess that may otherwise occur with catalyst nanoparticles. Thisapproach generates porous coatings selectively distributed on surfacesof catalyst supports having metal nanoparticles bound thereto, whichdecreases catalyst activity loss by suppressing aging caused bysintering. The current solution-based approach, relative to otherchemistry-based approaches, is a wet-chemistry process, which results ina higher thermal durability and reduces catalyst metal loadingrequirements, which can potentially lead to significant cost savings.The present technology can be self-limiting and may obstruct feweractive sites on catalyst particles relative to other coating methodsthat may entirely coat the surface of the catalyst metal nanoparticlesand/or to result in multiple coating layers, so that a potential loss inthe number of available catalyst active sites may occur.

In certain aspects, the coating of the present technology may beselectively deposited and may inhibit sintering in part by physicalseparation of adjacent catalyst metal nanoparticles. For example,relative to a conventional catalyst system having the same catalyst andsupport material, but lacking the porous coating, the present technologymay reduce a catalyst metal loading requirement by greater than or equalto about 30%, greater than or equal to about 40%, greater than or equalto about 50%, greater than or equal to about 60%, greater than or equalto about 70%, greater than or equal to about 80% or great than or equalto about 90%, such as from about 30% to about 90%, from about 40% toabout 80%, from about 50%, to about 80%, from about 60% to about 80%, orfrom about 70% to about 80%. In other aspects, relative to aconventional catalyst system having the same catalyst and supportmaterial, but lacking the porous coating, the present technology mayreduce a lightoff temperature by greater than or equal to about 10° C.,optionally greater than or equal to about 20° C., and in certainvariations, optionally reduce a lightoff temperature from greater thanor equal to about 30° C.

Accordingly, the present technology provides a method for preparing acatalyst through a solution-based approach. The method may includebinding at least one catalyst to a catalyst support. The catalyst may bea nanoparticle. The catalyst optionally comprises one or more platinumgroup metals (PGM), noble group metals, or the like. For example, thecatalyst may comprise one or more platinum group metals, such asruthenium (Ru), rhodium (Rh), palladium (Pd), osmium (Os), iridium (Ir),platinum (Pt), or combinations thereof noble metals, such as ruthenium(Re), copper (Cu), silver (Ag), gold (Au), or combinations thereof orother metals, such as iron (Fe), nickel (Ni), manganese (Mn), sodium(Na), potassium (K), magnesium (Mg), calcium (Ca), barium (Ba), orcombinations thereof. For example, the catalyst particle optionallycomprises a metal selected from the group consisting of: platinum (Pt),ruthenium (Ru), rhodium (Rh), palladium (Pd), osmium (Os) nanoparticle,iridium (Ir), gold (Au), iron (Fe), nickel (Ni), manganese (Mn), andcombinations thereof. In one variation, the catalyst particle maycomprise platinum (Pt), palladium (Pd), or mixtures thereof.

The catalyst particle may have a maximum diameter of greater than orequal to about 2 nm to less than or equal to about 10 nm, such as adiameter of about 2 nm, about 3 nm, about 4 nm, about 5 nm, about 6 nm,about 7 nm, about 8 nm, about 9 nm, or about 10 nm.

The catalyst support may comprise a metal oxide. The catalyst supportmay be in the form of a plurality of particulates (e.g., a powder) or amonolith structure (e.g., a honeycomb structure) that may be coated witha washcoat layer that includes the catalyst material. In certainvariations, the metal oxide may be selected from the group consistingof: cerium oxide (CeO₂), aluminum oxide/alumina (Al₂O₃), zirconium oxide(ZrO₂), titanium dioxide (TiO₂), silicon dioxide (SiO₂), magnesium oxide(MgO), zinc oxide (ZnO), barium oxide (BaO), potassium oxide (K₂O),sodium oxide (Na₂O), calcium oxide (CaO), lanthanum oxide (La₂O₃), andcombinations thereof. The catalyst support may further comprise dopants.The catalyst support may comprise dopants selected from the groupconsisting of barium (Ba), cerium (Ce), lanthanum (La), phosphorus (P),and combinations thereof.

If the catalyst support is a monolith structure that includes a washcoatlayer, the monolith structure may be formed from any the metal oxidesdiscussed above or zeolites. The washcoat layer may include the same ordifferent metal oxides to form a porous ceramic layer including thecatalyst material. The washcoat precursor including the catalystmaterial(s) can be applied to a surface of the catalyst support and thenheat treated, e.g., calcined to form a porous ceramic washcoat layerincluding the catalyst material dispersed therein.

In other aspects, the support may be provided in a plurality ofparticles (e.g., powder). In such variations, the support may have anaverage particle diameter of greater than or equal to about 0.8 μm toless than or equal to about 5 μm, greater than or equal to 1 μm to lessthan or equal to about 4 μm, greater than or equal to 1.5 μm to lessthan or equal to about 3.5 μm, or greater than or equal to 2 μm to lessthan or equal to about 3 μm, such as a diameter of about 0.8 μm, 1 μm,1.5 μm, 2 μm, 2.5 μm, 3 μm, 3.5 μm, 4 μm, 4.5 μm, or 5 μm. In certaininstances, the surface of the catalyst support may have a potential zerocharge (“PZC”) of less than 7.

After binding a catalyst particle to a catalyst support, also referredto herein as a “supported particle,” the method may comprise washing thesupported catalyst particle with an acidic solution. Washing may improvethe uniformity of the catalyst particle size where a plurality ofcatalyst particles is disposed on the catalyst support surface. Incertain aspects, the solution is aqueous. The acidic solution maycomprise acetic acid (CH₃COOH), nitric acid (HNO₃), and/or citric acid(C₆H₈O₇), by way of non-limiting example. Such a washing process may beparticularly suitable where the catalyst support and catalyst particlesare further pretreated in a reduction reaction.

Accordingly, in certain instances, the method may further includepretreating the supported particle system. For example, the supportedparticle system may be treated in a reducing atmosphere to promoteconversion of the metal(s) in the catalyst particle to a metallic state.In reducing the surface of the catalyst particle, in subsequentdeposition steps, the surface of the particle has minimal surfaceoxygen/hydroxides, thus rendering it more hydrophobic so that ittherefore remains uncoated and exposed. It should be noted that certaincatalyst particles may not require pre-treatment in a reducingatmosphere as they naturally have less oxidation and active oxygen onthe surface that could react in subsequent treatment steps. However, incertain variations, the method optionally includes pretreating, whichmay include disposing the support having the surface comprising thecatalyst particle in a reducing atmosphere to promote conversion of thecatalyst particle to a metallic state.

In one variation, the supported particle may be disposed in a reducingatmosphere while placed in a furnace or other vessel that may be sealedand heated. The furnace may be purged with a reducing atmosphere. Forexample, the furnace may be purged with a gas mixture comprising aninert gas and hydrogen (H₂). The gas mixture may comprise less than orequal to about 3% by volume hydrogen (H₂) with a balance being the inertgas. For example, the inert gas may be argon (Ar), nitrogen (N₂), orother inert gases. In one variation, the gas mixture may comprise argon(Ar) having less than or equal to about 3% by volume hydrogen (H₂). Suchpurging may occur for greater than or equal to about 30 minutes. Forexample only, in certain instances, the purging may occur for about 1hour. A flow rate of the gas used for purging may be greater than orequal to about 1 standard cubic feet per hour (SCFH). In other aspects,the supported particle system may be further heated in the furnacehaving a temperature of greater than or equal to about 200° C. and lessthan or equal to about 500° C. For example only, in certain instances,the supported particle system may be further heated in the furnacehaving a temperature of about 400° C. The rate of heating of thesupported particle system may occur at a rate of less than or equal toabout 20° C. per minute. The supported particle system may be maintainedat the temperature for greater than or equal to about 30 minutes. Inother instances, the supported particle system may be maintained at thetemperature for greater than or equal to about 2 hours. The supportedparticle system may then be cooled to ambient conditions, for example,to room temperature. After the optional reduction step of the activecatalyst particle in the supported particle system, a porous coating maythen be formed on a surface of the support.

Therefore, the method may further include contacting the supportedparticle with a solution comprising a metal salt dissolved in a solvent.In certain instances, the solution has an acidic pH. For example only,the solution may have a pH of less than or equal to about 6. In otherinstances, the solution may have a pH of less than or equal to about 5.In certain instances, the surface of the catalyst support in solutionmay be positively charged. The metal salt may comprise an elementselected from the group consisting of: aluminum (Al), cerium (Ce),zirconium (Zr), titanium (Ti), silicon (Si), magnesium (Mg), zinc (Zn),sodium (Na), potassium (K), barium (Ba), calcium (Ca), and combinationsthereof. In other instances, the metal salt may have a molarconcentration from greater than or equal to about 0.1M to less than orequal to about 1M in the solution. The concentration of the metal saltmay be dependent upon the PGM loading requirements and the size of thesupport. Additionally, the metal salt may have a concentration ofgreater than or equal to about 1 wt. % to less than or equal to about 50wt. % relative to the weight of the catalyst support after it isdeposited, as described further herein.

As non-limiting examples, suitable salts of Al include AlCl₃, Al(NO₃)₃,Al(OH)₃, Al₂(SO₄)₃, Al(ClO₃)₃, AlPO₄, and Al(PO₃)₃; salts of Ce includeCe(NO₃)₃, Ce(OH₄), Ce₂(SO₄)₃, and Ce(SO₄)₂; salts of Zr includeZr(HPO₄)₂, Zr(OH)₄, and Zr(SO₄)₂; salts of Ti include TiOSO₄ and TiOPO₄;salts of Si include SiPO₄(OH); salts of Mg include MgSO₄, Mg(NO₃)₂,MgHPO₄, and Mg₃(PO₄)₂; salts of Zn include Zn(NO₃)₂, Zn₃(PO₄)₂, andZnSO₄; salts of Ba include BaCO₃, BaCl₂, and BaCrO₄; salts of K includeKHSO₄, KCl, K₂CO₃, K₂CrO₄, K₂Cr₂O₇, KOH, KIO₃, KI, K₂MnO₄, KVO₃, K₂MoO₄,KNO₃, KClO₄, K₂S₂O₈, K₂HPO₄, K₄P₂O₇, and K₂SO₄; salts of Na includeNaBr, NaCl, Na₂CO₃, Na₂CrO₄, HCOONa, NaHSO₄, NaOH, NaBO₂, Na₂O₃Si,NaVO₃, Na₂MoO₄, NaNO₃, NaOOCCOONa, NaMnO₄, Na₃PO₄, Na₂HPO₄, Na₂H₂P₂O₇,Na₄P₂O₇, Na₂SO₄, and Na₃P₃O₉; salts of Ca include CaCl₂, CaCO₃, CaFPO₃,Ca(OH)₂, Ca(IO₃)₂, Ca(NO₃)₂, Ca(NO₂)₂, CaC₂O₄, Ca(H₂PO₄)₂, Ca₂P₂O₇, andCaSO₄; and any combinations of these salts may be employed in thesolution. In one variation, the metal salt may comprise Al and beselected from the group consisting of: AlCl₃, Al(NO₃)₃, Al(OH)₃,Al₂(SO₄)₃, Al(ClO₃)₃, AlPO₄, Al(PO₃)₃, and combinations thereof. Thesolvent is non-limiting, and can be water, an alcohol, or other organicsolute. In certain aspects, the solution is aqueous. The pH of thesolution may be maintained by adding diluted acid. For example, incertain instances, the pH of the solvent may be maintained by addingdiluted HNO₃.

Next, the method comprises contacting a support having a surfacecomprising a catalyst particle with a solution comprising a metal saltand having an acidic pH. In this manner, the metal salt can precipitateonto the surface of the support and is selectively applied to form aporous coating of metal oxide on the supported particle system. A timemay lapse between the reduction treatment and the instance when thesupported particle is brought into contact with the solution comprisingthe metal salt and having an acidic pH. In certain instances, a week ormore may elapse between the pretreatment and the instance when thesupported particle is brought into contact with the solution comprisingthe metal salt and having an acidic pH. In certain aspects, thecontacting of the surface of support (comprising the catalyst particle)with the solution includes submerging the surface in the solution. Themethod may further include applying ultrasound to the solution tofacilitate the precipitating and/or mixing the solution with a planetarycentrifugal mixer to facilitate the precipitating.

Depositing the metal salts on the surface of the support may compriseagitating the supported particle and the solution comprising the metalsalt, for example, by applying ultrasound for greater than or equal toabout 5 minutes to facilitate precipitation. For example only, in someinstances, the supported particle and the solution comprising the metalsalt may be agitated by applying ultrasound for greater than or equal toabout 15 minutes and in certain other variations, for about 30 minutes.In certain instances, the supported particle and solution may be mixedusing a high speed planetary centrifugal mixing. For example, thesupported particle and solution may be mixed using the high speedplanetary centrifugal mixture for greater than or equal to about 5minutes. In certain instances, the metal salt will hydrolyze and becomenegatively charged. The negatively charged metal salts may bind in aself-limiting manner to the exposed surface of the catalyst support.After a predetermined duration of time, the supported particle systemmay be removed from the solution.

Applying the metal salts to the surface of the support may furthercomprise removing the solvent from the supported particle system havingthe liquid disposed thereon, such as, for example, by evaporation ordrying. In certain instances, evaporating or drying the solvent isperformed by increasing the temperature or decreasing the pressure nearthe supported particle in contact with the metal salt solution tofacilitate precipitation of the metal salt on the surface of thesupport. In certain instances, drying is performed at a temperature ofgreater than or equal to about 50° C. For example only, drying may beperformed at a temperature of about 80° C. Drying may be performed at apressure of less than or equal to about 1 Torr. Drying may be performedfor a time of greater than or equal to about 4 hours. For example only,drying may be performed for a time of about 6 hours. For example only,drying may occur in a vacuum oven where over a course of a hour thevacuum oven may be gradually heated to greater than or equal to about80° C. at a pressure of 1 Torr. After warming, the vacuum oven may bekeep at 80° C. for greater than or equal to about 6 hours. However, itis understood that other temperatures and durations may be used toremove solvent from support particles.

The method may further comprise calcining the metal salts afterprecipitation to generate a porous oxide coating on the support, whereinthe metal oxide is derived from the metal salt. In certain aspects, themethods of the present disclosure selectively generate a porous coatingof metal oxide on the surface of the support distributed around thecatalyst particle(s). The porous coating thus formed has a porosity,i.e., a volume of pores relative to the volume of coating, of greaterthan or equal to about 20% to less than or equal to about 70%, such as aporosity of about 20%, about 30%, about 40%, about 50%, about 60%, orabout 70%.

Calcining includes heating the catalyst particle, support, and metalsalts at a temperature of greater than or equal to about 400° C. to lessthan or equal to about 600° C., such as at a temperature of about 550°C., for a time of greater than or equal to about 2 hours to generate aporous coating of metal oxide nanoparticles on the support. In variousaspects, the temperature applied during calcining does not exceed themelting point of the metal oxide derived from the metal salt.Non-limiting examples of metal oxides formed from metal salts includeAl₂O₃, CeO₂, ZrO₂, TiO₂, SiO₂, MgO, ZnO, BaO, K₂O, Na₂O, CaO, andcombinations thereof.

With reference to FIG. 1, the current technology provides a catalystsystem 10 that resists sintering and retains catalytic activity afterprolonged exposures to elevated temperatures. The catalyst system 10 canbe a catalyst system generated by the method provided herein. Thecatalyst system 10 includes metal nanoparticles 12 (catalyst) bound to acatalyst metal oxide support 14 in the form of a monolith structure anda coating 16 of metal oxide nanoparticles 18 disposed on the metal oxidesupport 14. The coating 16 is intermittently and selectively disposed ona surface 20 of the metal oxide support 14 distributed around thecatalyst metal nanoparticles 12. In this manner, the porous coating 16leaves at least a portion of the surface(s) of the catalyst metalnanoparticles 12 exposed so that the active metal sites are availablefor reaction. In this manner, in certain aspects, the coating 16 avoidscovering the catalyst metal nanoparticles 12 or at least partiallyavoids covering the catalyst metal nanoparticles 12. In certaininstances, the catalyst metal nanoparticles 12 are either directly orindirectly coupled or bound to the metal oxide support 14.

The catalyst particles (e.g., metal nanoparticles 12) may have a loadingdensity on the catalyst support of greater than or equal to about 0.25%(w/w) to less than or equal to about 20% (w/w), such as a loadingdensity of about 0.25% (w/w), about 0.5% (w/w), about 1% (w/w), about1.5% (w/w), about 2% (w/w), about 2.5% (w/w), about 3% (w/w), about 3.5%(w/w), about 4% (w/w), about 4.5% (w/w), about 5% (w/w), about 5.5%(w/w), about 6% (w/w), about 6.5% (w/w), about 7% (w/w), about 7.5%(w/w), about 8% (w/w), about 8.5% (w/w), about 9% (w/w), about 9.5%(w/w), or about 10% (w/w). In certain instances, the loading density ofthe metal nanoparticles 12 on the metal oxide support 14 is about 1.5%(w/w).

As described above in regard to the method of preparing a catalyst, thenanoparticles 12 may comprise PGM nanoparticles, such as nanoparticlesof Ru, Rh, Pd, Os, Ir, or Pt, a noble metal, such as nanoparticles ofRe, Cu, Ag, Au, other metals such as nanoparticles of Fe, Ni, Mn, Na, K,Mg, Ca, or Ba, or combinations thereof.

As discussed above, the metal oxide support 14 optionally comprises ametal oxide selected from the group consisting of Al₂O₃, CeO₂, ZrO₂,TiO₂ SiO₂, MgO, ZnO, BaO, K₂O, Na₂O, CaO, and combinations thereof.Nonetheless, it is understood that this group of metal oxides is notlimited and that other metal oxides may be employed for the support 14.It should be noted that the catalyst support may be a monolith structureas shown in FIG. 1 or alternatively, the support shown in FIG. 1 may beone or more washcoat layers formed on a monolith support structure.

FIG. 2 shows the catalyst support in the form of a particle (which maybe provided as a plurality of particles or powder). A catalyst system 30that resists sintering and retains catalytic activity after prolongedexposures to elevated temperatures may thus be in the form of aparticle. Thus, a plurality of particles like the catalyst system 30 maybe combined in a conventional casting process to form a catalyst system.A plurality of metal nanoparticles 32 (catalyst) is bound to a catalystmetal oxide support 34 and a coating 36 of metal oxide nanoparticles 38is disposed on the metal oxide support 34. In certain instances, themetal nanoparticles 32 are either directly or indirectly coupled orbound to the metal oxide support 34. The coating 36 is intermittentlyand selectively disposed on a surface 40 of the metal oxide support 34distributed around the catalyst metal nanoparticles 32.

Where the support is in the form of a plurality of particles, thesupport particles may have an average diameter 42 of greater than orequal to about 0.8 μm to less than or equal to about 5 μm, greater thanor equal to 1 μm to less than or equal to about 4 μm, greater than orequal to 1.5 μm to less than or equal to about 3.5 μm, or greater thanor equal to 2 μm to less than or equal to about 3 μm, such as a diameterof about 0.8 μm, about 1 μm, about 1.5 μm, about 2 μm, about 2.5 μm,about 3 μm, about 3.5 μm, about 4 μm, about 4.5 μm, or about 5 μm. Itshould be noted the catalyst metal oxide support 14 may have shapes orforms other than a planar structure as shown in FIG. 1, for example, itmay have conventional monolith or honeycomb shapes or the catalystsupport may be in the form of beads for a packed bed catalyst. Moreover,the metal oxide support (either 14 or 34 in FIGS. 1 and 2) may have asurface area of greater than or equal to about 50 m²/g to less than orequal to about 150 m²/g, greater than or equal to about 75 m²/g to lessthan or equal to about 125, m²/g such as a surface area of about 75m²/g, about 80 m²/g, about 90 m²/g, about 100 m²/g, about 110, m²/gabout 120 m²/g, about 130 m²/g, about 140 m²/g, about 145 m²/g, or about150 m²/g.

The coating (either 16 or 36 in FIGS. 1 and 2) includes metal oxidenanoparticles (18 or 38), such as for example, Al₂O₃, CeO₂, ZrO₂, TiO₂,SiO₂, MgO, ZnO, BaO, K₂O, Na₂O, CaO, and combinations thereof. The metaloxide nanoparticles have an average maximum diameter (either 22 in FIG.1 or 44 in FIG. 2) of greater than or equal to about 0.5 nm to less thanor equal to about 50 nm, greater than or equal to about 1 nm to lessthan or equal to about 25 nm, or greater than or equal to about 2 nm toless than or equal to about 10 nm, such as a diameter of about 0.5 nm,about 1 nm, about 2 nm, about 3 nm, about 4 nm, about 5 nm, about 10 nm,about 15 nm, about 20 nm, about 25 nm, about 30 nm, about 35 nm, about40 nm, about 45 nm, or about 50 nm. Nonetheless, the coating (either 16or 36 in FIGS. 1 and 2) comprising the metal oxide nanoparticles (18 or38) may be crystalline.

The metal oxide nanoparticles 18 of the coating 16 may comprise greaterthan about 0% by weight to less than or equal to about 20% by weight ofthe total catalyst system weight (including the catalyst metal oxidesupport 14, the metal nanoparticles 12, and the coating 16). In certaininstances, the coating 16 may have a thickness of greater than or equalto about 1 mm. The coating 16 may comprise a single layer or multiplelayers. The coating 16 of metal oxide nanoparticles 18 may cover a largearea of otherwise exposed surface area of the metal oxide support. Thecoating 16 comprises a plurality of pores 22, i.e., is porous, such thatreacting gas molecules can access the catalyst metal nanoparticles 12having catalytic activity, yet coalescing of metal nanoparticles 12 withother the metal nanoparticles is minimized or prevented. Therefore, thecoating 16 may render the catalyst system 10 resistant to sintering orthermal degradation by increasing the surface area of the catalystsystem 10. In particular, by increasing the number of available surfacesites of the catalyst system 10, which can be expressed as catalyticmetal dispersion.

In certain instances, the nanoparticles (either 12 in FIG. 1 or 32 inFIG. 2) may have a maximum average diameter 24 in FIG. 1 or 46 in FIG. 2of greater than or equal to about 2 nm to less than or equal to about 10nm, such as a diameter of about 2 nm, about 3 nm, about 4 nm, about 5nm, about 6 nm, about 7 nm, about 8 nm, about 9 nm, or about 10 nm.

“Catalyst metal dispersion” refers to a ratio of metal catalyst surfacesites to a mass of an entire catalyst system. Therefore, a catalystsystem with a high dispersion will have smaller and more highlydispersed metal catalyst relative to a catalyst system with a lowdispersion. Relative to a catalyst system equivalent to the catalystsystem described herein, but without a porous coating, a catalyst systemhaving an increased resistance to sintering has a dispersion loss ofless than 74% after exposure to a temperature of about 650° C. for atime period of about 2 hours. A catalyst system that resists sinteringis a catalyst system that undergoes a dispersion loss of less than orequal to about 20%, less than or equal to about 15%, or less than orequal to about 10% after exposure to a temperature of about 650° C. fora time period of about 2 hours.

In certain instances, pores may be defined between metal oxidenanoparticles within the coatings (nanoparticles 18 in coating 16 inFIG. 1 or metal oxide nanoparticles 38 in coating 36 in FIG. 2) havingan average diameter of greater than or equal to about 0.5 nm to lessthan or equal to about 30 nm, such as a diameter of about 0.5 nm, about1 nm, about 2 nm, about 3 nm, about 4 nm, about 5 nm, about 6 nm, about7 nm, about 8 nm, about 9 nm, about 10 nm, about 11 nm, about 12 nm,about 13 nm, about 14 nm, about 15 nm, about 16 nm, about 17 nm, about18 nm, about 19 nm, or about 20 nm. The coating optionally has aporosity of greater than or equal to about 20% to less than or equal toabout 70%, such as a porosity of about 20%, about 30%, about 40%, about50%, about 60%, or about 70%.

In various embodiments, the coating may cover greater than or equal toabout 1.5% to less than or equal to about 80% of the exposed surfacearea of the catalyst support, or greater than or equal to about 30% toless than or equal to about 80% of the exposed regions of the surface ofthe catalyst support. The exposed surfaces of the catalyst support referto the portions of the catalyst support surface to which catalyst metalparticles are not bound. In various embodiments, the coating coversabout 40%, about 50%, about 60%, about 70%, or about 80% or more of theexposed surface area of the catalyst support. A total amount of surfacearea coverage of the support including both the catalyst metal particlesand metal oxide nanoparticles (forming the coatings) is greater than orequal to about 30% to less than or equal to about 80%.

In certain instances, the metal oxide nanoparticles (18 of FIG. 1 or 38of FIG. 2) of the coating (16 of FIG. 1 or 36 of FIG. 2) comprise thesame metal oxide composition as the metal oxide support (14 of FIG. 1 or34 of FIG. 2) composition. In other instances, the metal oxidenanoparticles of the coating comprise a different metal oxide(s) thanthe support. In yet other instances, the coating comprises a pluralityof different metal oxides. Therefore, the catalyst system may include asingle species of metal particles and metal oxide support or a pluralityof metal particles and metal oxide supports.

Embodiments of the present technology are further illustrated throughthe following non-limiting examples.

Example 1

A powder of supported nanoparticles comprising platinum group metals(PGM—comprising Pt—Pd) nanoparticles bound to an Al₂O₃ (alumina) isfirst pretreated in a reducing environment. A furnace is purged withAr+3% H₂ for about 1 hour at a flow rate of 1 SCFH. The supportednanoparticle is heated to about 400° C. at a heating rate of less thanor equal to about 10° C. per minute. The supported nanoparticles aremaintained at about 400° C. for greater than 2 hours and then cooled toroom temperature with gas flowing.

An aqueous solution is formed by dissolving Al(NO₃)₃ in water. Thesolution pH is maintained at less than or equal to about 5. The reducedsupported nanoparticle gradually contacts the metal salt solution. Thesupported nanoparticle and the solution are mixed using anultrasonicator for about 5 minutes. The mixture is gradually heated toabout 80° C. over a course of an hour. The mixture is maintained at 80°C. for greater than or equal to about 6 hours and calcined at atemperature that is greater than or equal to about 500° C. and less thanor equal to about 550° C. for greater than about 2 hours to generate aporous alumina coating over the catalyst metal oxide support particles.

The coated-supported nanoparticles and a control of supportednanoparticles without a coating are subjected to aging conditions byheating to about 850° C. for about 2 hours in air having 10% water. Toevaluate activity, CO and C₃H₆ oxidation reactions are used (CO+O₂;C₃H₆+O₂). In one instance, a stream of 5000 ppm CO, 1% O₂, 5% H₂O isflowed over the catalyst as reaction temperature is increased. In theother instance, a stream of 500 ppm C₃H₆, 1% O₂, 5% H₂O is flowed overthe catalyst as reaction temperature is increased In both instances, theramping rate is 2° C. per/minute from about 100° C. to about 350° C. Thetotal flow rate is 0.5 L/minute with a balance consisting of N₂ gas. Theamount of CO and C₃H₆ is detected post-catalyst is measured to evaluatethe extent of the reaction. As seen in FIG. 3, a metric used to evaluateactivity is T₅₀ (lightoff temperature), which is the temperature atwhich 50% of the CO and C₃H₆ streams are being oxidized over thecatalyst, respectively. After subjecting the control of supportednanoparticles 50 and the coated-supported nanoparticles 52 the gainingconditions described above with respect to the CO oxidation reaction 54,the control of the supported nanoparticles 50 and the coatednanoparticles 52 provided T₅₀ 58 values of 188° C. and 172° C.,respectively. After subjecting the control of supported nanoparticles 50and the coated-supported nanoparticles 52 the gaining conditionsdescribed above with respect to the C₃H₆ oxidation reaction 56, thecontrol of the supported nanoparticles 50 and the coated nanoparticles52 provided T₅₀ 58 values of 224° C. and 194° C., respectively. It isdesirable to have low T₅₀ 58 values.

Example 2

A general washcoat slurry batching procedure is used to producelaboratory reactor test samples. These samples can range in size fromless than 1 in³ to about 10 in³, and require relative small quantitiesof washcoat slurry to achieve dry washcoat loadings in the range of1.5-3.5 g/in³.

A ball mill jar with a free capacity of 0.5 L and cylindrical aluminamilling media (12 mm diameter×12 mm long) are selected for use. Thecylindrical alumina milling media are placed into a ball mill jar havinga free capacity. About 60% of the ball mill jar is filled with thealumina milling media. For example, the ball mill jar may have acapacity of about 0.5 L, and the alumina milling media may have adiameter of about 12 mm and a length of about 12 mm. Therefore, thealumina milling media may equal about 625 g or 0.3 L. The volume of thematerial to be milled, in aqueous suspension, desirably covers the millmedia by no more than about 10% of the mill jar volume. The volume ofthe material charged may be equal to roughly the void volume of themedia charge plus 10% of the jar's volume. The final slurry may have amilled washcoat density of greater than or equal to about 1 to less thanor equal to about 1.5 g/mL and target solids fractions (F_(s)) ofgreater than or equal to about 0.45 and less than or equal to about0.50. The water and the dry PGM alumina catalyst will each have a massof 90 g in the final slurry.

After ball milling for about 30 minutes, the slurry is washcoated ontomonolith core samples having a diameter of about three-quarters of aninch, a length of greater than or equal to about 1 inch with about 400cells per square (“CPSI”) with about a 4 mill-inch wall thickness. Afterwashcoating, the monolithic catalyst is dried at about 120° C. andcalcined at a temperature that is greater than or equal to about 500° C.and less than or equal to about 550° C. for greater than about 2 hoursin first static air to generate a porous alumina coating over thecatalyst metal oxide supports.

Example 3

FIG. 3 shows lightoff temperatures 58 (° C.) versus for a comparativecontrol catalytic system with a Pt—Pd catalyst on Al₂O₃ support havingno coating (50) and coated-supported nanoparticles (52) prepared inaccordance with certain principles of the present disclosure asdescribed in Example 1. The comparative catalytic systems are subjectedto aging conditions by heating to about 850° C. for about 2 hours in airhaving 10% water. To evaluate activity, CO and C₃H₆ oxidation reactionsare used (CO+O₂; C₃H₆+O₂). FIG. 3 shows CO lightoff temperatures 54 andC₃H₆ lightoff temperatures 56, where the lower the lightoff temperature,the better the performance of the catalytic system. In one instance, astream of 5000 ppm CO, 1% O₂, 5% H₂O is flowed over the catalyst asreaction temperature is increased. In the other instance, a stream of500 ppm C₃H₆, 1% O₂, 5% H₂O is flowed over the catalyst as reactiontemperature is increased. In both instances, the ramping rate is 2° C.per/minute from about 100° C. to about 350° C. The total flow rate is0.5 L/minute with a balance consisting of N₂ gas. The amount of CO andC₃H₆ is detected post-catalyst is measured to evaluate the extent of thereaction in either instance, (i.e., coated and non-coated supportednanoparticles). Thus, the inventive coated-supported nanoparticles 52show lower lightoff temperatures and improved catalytic performance ascompared to the control 50 for both CO and C₃H₆.

Example 4

As seen in FIG. 4, another metric used to evaluate activity is T₅₀,which is the lightoff temperature at which 50% of the CO and C₃H₆streams are being oxidized over the catalyst, respectively. Acomparative control catalytic system with a Pt—Pd catalyst on Al₂O₃support having no coating (60) and coated-supported nanoparticles (62)are prepared in accordance with certain principles of the presentdisclosure as described in Example 1. After subjecting the control ofsupported nanoparticles 60 and the coated-supported nanoparticles 62 tothe gaining conditions described above with respect to the CO oxidationreaction 64, the control of the supported nanoparticles 60 and thecoated nanoparticles 62 provided T₅₀ 68 values of 156° C. and 150° C.,respectively. After subjecting the control of supported nanoparticles 60and the coated-supported nanoparticles 62 to the gaining conditionsdescribed above with respect to the C₃H₆ oxidation reaction 66, thecontrol of the supported nanoparticles 60 and the coated-supportednanoparticles 62 provided T₅₀ 68 values of 168° C. and 160° C.,respectively.

Example 5

As seen in FIG. 5, another metric used to evaluate activity is T₉₀,which is the lightoff temperature at which 90% of the CO and C₃H₆streams are being oxidized over the catalyst, respectively. Acomparative control catalytic system with a Pt—Pd catalyst on Al₂O₃support having no coating (70) and coated-supported nanoparticles (72)are prepared in accordance with certain principles of the presentdisclosure as described in Example 1. After subjecting the control ofsupported nanoparticles 70 and the coated-supported nanoparticles 72 thegaining conditions described above with respect to the CO oxidationreaction 74, the control of the supported nanoparticles 70 and thecoated nanoparticles 72 provided T₉₀ 78 values of 171° C. and 164° C.,respectively. After subjecting the control of supported nanoparticles 70and the coated-supported nanoparticles 72 the gaining conditionsdescribed above with respect to the C₃H₆ oxidation reaction 76, thecontrol of the supported nanoparticles 70 and the coated nanoparticles72 provided T₉₀ 78 values of 182° C. and 171° C., respectively. It isdesirable to have low T₅₀ 68 and T₉₀ 78 values.

The foregoing description has been provided for purposes of illustrationand description. It is not intended to be exhaustive or to limit thedisclosure. Individual elements or features of a particular embodimentare generally not limited to that particular embodiment, but, whereapplicable, are interchangeable and can be used in a selectedembodiment, even if not specifically shown or described. The same mayalso be varied in many ways. Such variations are not to be regarded as adeparture from the disclosure, and all such modifications are intendedto be included within the scope of the disclosure.

What is claimed is:
 1. A catalyst system comprising: a platinum groupmetal catalyst bound to a metal oxide support; and a crystalline coatingof metal oxide nanoparticles selectively disposed on the metal oxidesupport, wherein the crystalline coating has a porosity of greater thanabout 20% to less than about 70% and covers greater than or equal toabout 1.5% to less than or equal to about 80% of an exposed surface areaof the metal oxide support.
 2. The catalyst system according to claim 1,wherein the platinum group metal catalyst comprises a metal selectedfrom the group consisting of: platinum (Pt), ruthenium (Ru), rhodium(Rh), palladium (Pd), osmium (Os), iridium (Ir), gold (Au), andcombinations thereof.
 3. The catalyst system according to claim 1,wherein the metal oxide nanoparticles comprise an element selected fromthe group consisting of: aluminum (Al), cerium (Ce), zirconium (Zr),titanium (Ti), silicon (Si), magnesium (Mg), zinc (Zn), sodium (Na),potassium (K), barium (Ba), calcium (Ca), and combinations thereof. 4.The catalyst system according to claim 1, wherein the metal oxidenanoparticles comprise a metal oxide selected from the group consistingof: aluminum oxide (Al₂O₃), cerium oxide (CeO₂), zirconium oxide (ZrO₂),titanium dioxide (TiO₂), silicon dioxide (SiO₂), magnesium oxide (MgO),zinc oxide (ZnO), barium oxide (BaO), potassium oxide (K₂O), sodiumoxide (Na₂O), calcium oxide (CaO), and combinations thereof.
 5. Thecatalyst system according to claim 1, wherein the metal oxide supportcomprises a metal oxide selected from the group consisting of: ceriumoxide (CeO₂), aluminum oxide (Al₂O₃), zirconium oxide (ZrO₂), titaniumdioxide (TiO₂), silicon dioxide (SiO₂), magnesium oxide (MgO), zincoxide (ZnO), barium oxide (BaO), potassium oxide (K₂O), sodium oxide(Na₂O), calcium oxide (CaO), lanthanum oxide (La₂O₃), and combinationsthereof; and the metal oxide nanoparticles comprise aluminum oxide(Al₂O₃).
 6. The catalyst system according to claim 1, wherein the metaloxide support comprises aluminum oxide (Al₂O₃) and the metal oxidenanoparticles comprise aluminum oxide (Al₂O₃).
 7. The catalyst systemaccording to claim 1, wherein a loading density of the platinum groupmetal catalyst on the metal oxide support is greater than or equal toabout 0.25% (w/w) to less than or equal to about 20% (w/w).
 8. Thecatalyst system according to claim 1, wherein the metal oxide support isa monolith structure or a particle.
 9. The catalyst system according toclaim 1, wherein the metal oxide nanoparticles have an average diameterof greater than or equal to about 0.5 nm to less than or equal to about50 nm and the crystalline coating comprises pores having an averagediameter of greater than or equal to about 0.5 nm to less than or equalto about 30 nm.
 10. A catalyst system comprising: a platinum group metalcatalyst selected from the group consisting of: palladium, platinum, andcombinations thereof bound to a metal oxide support; and a crystallinecoating of metal oxide nanoparticles selectively disposed on the metaloxide support distributed around the platinum group metal catalyst,wherein the crystalline coating has a porosity of greater than about 20%to less than about 70% and covers greater than or equal to about 1.5% toless than or equal to about 80% of an exposed surface area of the metaloxide support.
 11. The catalyst system according to claim 10, whereinthe metal oxide nanoparticles comprise a metal oxide selected from thegroup consisting of: aluminum oxide (Al₂O₃), cerium oxide (CeO₂),zirconium oxide (ZrO₂), titanium dioxide (TiO₂), silicon dioxide (SiO₂),magnesium oxide (MgO), zinc oxide (ZnO), barium oxide (BaO), potassiumoxide (K₂O), sodium oxide (Na₂O), calcium oxide (CaO), and combinationsthereof.
 12. The catalyst system according to claim 10, wherein themetal oxide support comprises a metal oxide selected from the groupconsisting of: cerium oxide (CeO₂), aluminum oxide (Al₂O₃), zirconiumoxide (ZrO₂), titanium dioxide (TiO₂), silicon dioxide (SiO₂), magnesiumoxide (MgO), zinc oxide (ZnO), barium oxide (BaO), potassium oxide(K₂O), sodium oxide (Na₂O), calcium oxide (CaO), lanthanum oxide(La₂O₃), and combinations thereof; and the metal oxide nanoparticlescomprise aluminum oxide (Al₂O₃).
 13. The catalyst system according toclaim 10, wherein the metal oxide support comprises aluminum oxide(Al₂O₃) and the metal oxide nanoparticles comprise aluminum oxide(Al₂O₃).
 14. The catalyst system according to claim 10, wherein aloading density of the platinum group metal catalyst on the metal oxidesupport is greater than or equal to about 0.25% (w/w) to less than orequal to about 20% (w/w).
 15. The catalyst system according to claim 10,wherein the metal oxide support is a monolith structure or a particle.16. The catalyst system according to claim 10, wherein the metal oxidenanoparticles have an average diameter of greater than or equal to about0.5 nm to less than or equal to about 50 nm and the crystalline coatingcomprises pores having an average diameter of greater than or equal toabout 0.5 nm to less than or equal to about 30 nm.
 17. A catalyst systemcomprising: a platinum group metal catalyst selected from the groupconsisting of: palladium, platinum, and combinations thereof bound to ametal oxide support; and a crystalline coating of metal oxidenanoparticles that comprise aluminum oxide (Al₂O₃) selectively disposedon the metal oxide support comprising aluminum oxide (Al₂O₃), whereinthe metal oxide nanoparticles have an average maximum diameter ofgreater than or equal to about 0.5 nm to less than or equal to about 50nm and the crystalline coating has a porosity of greater than about 20%to less than about 70% and covers greater than or equal to about 1.5% toless than or equal to about 80% of an exposed surface area of the metaloxide support.
 18. The catalyst system according to claim 17, wherein aloading density of the platinum group metal catalyst on the metal oxidesupport is greater than or equal to about 0.25% (w/w) to less than orequal to about 20% (w/w).
 19. The catalyst system according to claim 17,wherein the metal oxide support is a monolith structure or a particle.